SENSITIVE INDUCTION OF APOPTOSIS IN BREAST-CANCER CELLS BY A NOVEL 1,25-DIHYDROXYVITAMIN-D-3 ANALOG SHOWS RELATION TO PROMOTER SELECTIVITY

The biologically active form of vitamin D-3, the nuclear hormone 1 alp ha,25-dihydroxyvitamin D-3 (VD), is an important regulator of cellular growth, differentiation, and death. The hormone mediates its action t hrough the activation of the transcription factor VDR, which is a memb er of the superfamily of nuclear receptors. In most cases the ligand-a ctivated VDR is found in complex with the retinoid X receptor (RXR) an d stimulates gene transcription mainly from VD response elements (VDRE s) that are formed by two hexameric core binding motifs and are arrang ed either as a direct repeat spaced by three nucleotides (DR3) or as a n inverted palindrome spaced by nine nucleotides (IP9). The two VD ana logues CB1093 and EB1089 are both very potent inhibitors of the prolif eration of MCF-7 cultured breast cancer cells displaying approximately 100-fold lower IC50 values (0.1 nM) than the natural hormone. In addi tion, CB1093 is even more potent in vivo than EB1089 in producing regr ession of experimental mammary tumors. Moreover, both VD analogues ind uce apoptosis in MCF-7 cells, but CB 1093 is effective at concentratio ns approximately 10-fold lower than EB1089. In accordance, the reducti on of Bcl-2 protein expression showed CB1093 to be more potent than EB 1089. This suggests that the antiproliferative effect of CB1093 may be related mainly to its apoptosis inducing effect whereas EB1089 may pr eferentially have effects on growth arrest. EB1089 is known to result in a selectivity for the activation of IP9-type VDREs, whereas CB1093 shows a preference for the activation of DR3-type VDREs. This promoter selectivity suggests that the effects of VD and its analogues on grow th arrest and the induction of apoptosis may be mediated by different primary VD responding genes. In conclusion, CB1093 was found to be a p otent inhibitor of rat mammary tumor growth in vivo. CB1093 also displ ayed a high potency in vitro in the induction of apoptosis, a process that may be linked to a promoter selectivity for DR3-type VDREs. (C) 1 997 Wiley-Liss, Inc.